Optical disc tracking system with switching of tracking error signals at boundary between track guide and track address

ABSTRACT

A tracking system for positioning a light spot onto a magneto-optic disc having a beam guide track preformatted thereon, includes a main light beam and two auxiliary light beams, and a circuit for switching between a low frequency component of a tracking error signal, detected by respective auxiliary light beam detectors, and a tracking error signal detected by a main light beam detector.

BACKGROUND OF THE INVENTION

1. (Field of Technology)

The present invention relates to a tracking system for an optical memorydevice of a type wherein any of the information recording, reproductionand erasing can be carried out by radiating a laser beam to a memorymedium.

2. (Description of the Prior Art)

Optical memory devices have recently drawn the attention of people as ahigh density, high capacity memory device. The reason that an opticalmemory device is of high density and high capacity is because the sizeof each bit which represents a unit of information storage can bereduced to a diameter of about 1 μm. This, in turn, however, imposessome limitations on the optical memory device. More specifically, inorder for information to be recorded on, or reproduced from, apredetermined location, the light beam is required to be accuratelypositioned.

Because of the foregoing, when using a disc capable of recordinginformation simultaneously with erasure of previously recordedinformation, it is a general practice for the disc substrate to bepermanently provided with beam guide tracks or address information.

The guide tracks generally have a shape as shown in FIG. 10 of theaccompanying drawings and are in the form of grooves of a depthgenerally equal to the wavelength λ divided by the product of therefractive index n times 8, i.e., λ/8n. Any of the informationrecording, reproduction and erasing is carried out while the light beamscans the tracks of the disc guided along these guide grooves.

As a means for sensing a tracking signal from the guide grooves, twomethods are well-known; a Twin Spot method (a three-beam method) such asgenerally used in association with VD (video disc) and CD (compact disc)and a push-pull method such as generally used in association with anoptically writeable disc. The Twin Spot method and the push-pull methodare illustrated respectively in FIGS. 12 and 13 of the accompanyingdrawings.

The Twin Spot method has an advantage in that a stable trackingperformance can be achieved even though a pick up is inclined relativeto the optical disc substrate. However, it has a problem in that, when atracking beam scans a boundary between a guide groove region G and anaddress information region A constituted by a plurality of pits as shownin FIG. 12(a), the tracking tends to be disturbed because of thedifference between a diffraction efficiency on the leading beam B₁ andthat on the trailing beam B₂. It is to be noted that reference characterR used in FIG. 12(b) represents recorded bits.

On the other hand, although the push-pull method is generally free fromthe above mentioned problem inherent in the Twin Spot method because ofthe tracking performed by a single beam B₄ as shown in FIG. 13(a), ithas a problem in that, because the position of the light beam which hasbeen reflected towards a detector D shown in FIG. 13(b) tends to becomedisplaced relative thereto in the event of a shift in position of a lensas a result of tracking or in the event of inclination of the pick-uprelative to the disc, the tracking error signal tends to accompany asteady drift which will bring about a continuing shift in tracking.Accordingly, in the event that the pick-up has inclined relative to thedisc, the pattern of diffraction occurring at the guide groove regionand that at the address information region differ from each other and,as a result thereof, the amount of tracking shift necessarily deviatesto such an extent as to result in a disturbed tracking at the boundary.FIGS. 12(a) to 12(c) and FIGS. 13(a) to 13(c) are schematicrepresentations illustrative of the change in tracking error signaloccurring during the tracking at the boundary according to these twomethods, respectively.

In these figures, the servo region is considered to be sufficientlylower than the pit reproducing frequency and, therefore, an output ofthe detector during the tracking at the address information region isshown as an average value. FIG. 12 applies where the difference inamount of beam reflected is taken as a tracking error signal, whereasFIG. 13 applies where the difference in output from detectors fordetecting two split beam components is taken as the tracking errorsignal.

SUMMARY OF THE INVENTION

Accordingly, the present invention has been devised to substantiallyeliminate the above described problems and has for its essential objectto provide a tracking system wherein a relatively easy signal processingmeans is employed to enable tracking to be performed in a stabilizedmanner, even to discontinuous guide grooves, with no need to takespecial measures for any disc substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

This and other objects and features of the present invention will becomeclear from the following description taken in conjunction with preferredembodiments thereof with reference to the accompanying drawings, inwhich:

FIG. 1 is a schematic diagram showing an optical head for amagneto-optic disc according to one embodiment of the present invention;

FIG. 2 is a diagram showing an electric circuit forming a detectingmeans for detecting a tracking error signal;

FIG. 3 is a circuit block diagram showing the circuit of the detectingmeans;

FIG. 4 (a,b) is a chart showing waveforms of signals appearing in thecircuit of FIG. 3;

FIG. 5 is a schematic diagram showing an optical head for an opticaldisc to which a tracking system is applied according to a secondembodiment of the present invention;

FIG. 6 is a schematic diagram showing a relationship between therespective positions of laser beam spots incident on the optical disc,the arrangement of detecting elements of a photo-detector and therespective positions of the laser beam spots reflected toward thedetecting elements;

FIG. 7 is a circuit block diagram showing a circuit used to effect aprocessing necessary to produce a feedback control signal;

FIGS. 8(a,b) are charts showing waveforms of signals;

FIG. 9 is a diagram similar to FIG. 7 showing a modified form of thecircuit;

FIGS. 10(a,b,c) are charts showing waveforms of signals;

FIG. 11 is a perspective view of a portion of a disc substrate;

FIGS. 12(a,b,c) are plan views of a portion of the disc substrate ontowhich the laser beams are projected;

FIG. 13(a) is a plan view of a portion of the disc substrate onto whichthe laser beams are projected;

FIG. 13(b) is a schematic diagram showing the detector on which the beamspot is reflected; and

FIG. 13(c) is a diagram showing waveforms of the signals.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIGS. 1 and 2 illustrate the structure of an optical head for amagneto-optic disc and a circuit forming a detecting means for detectinga tracking error signal outputted form the optical head, respectively.

Referring first to FIG. 1, reference numeral 1 represents amagneto-optic disc comprising a disc substrate which contains guidegrooves and address information and is coated with a magnetizable layerhaving an anisotropy of magnetism in a vertical direction. A laser beamproduced from a semiconductor laser device 2 travels through acollimator lens 3 and then through a shaping prism 4 by which theelliptical cross-sectional representation of the laser beam istransformed into a generally circular shape. Reference numeral 5represents a diffraction grating for splitting the laser beam,diffracted light of a spectral degree of 0 and ±1 being utilized. Apolarizing beam splitter 6 is disposed between the diffraction grating 5and an objective lens 8 for projecting a tiny spot of laser beam ontothe magnetizable layer of the magneto-optic disc after having beendeflected 90° by a total reflecting mirror 7. This polarizing beamsplitter 6 serves to improve the polarization ratio of the incominglight and to further rotate the plane of polarization of the light,reflected from the disc, for generally increasing the angle ofmagneto-optic rotation. This beam splitter 6 also serves to guide aportion of the laser beam toward photo-detectors 11, 12 and 17 as willbe described later.

The objective lens 8 referred to above is in practice driven by a servocontrol circuit (not shown) so that the size and position of the lightspot projected onto the information tracks of the magnetizable layer ofthe disc can be adjusted. Reference numeral 9 represents a polarizingbeam splitter having, as is the case with the beam splitter 6, afunction of increasing the angle of magneto-optic rotation with respectto the reflected light. Reference numeral 10 represents a spot lens forprojecting the information light, reflected from the beam splitter 9,onto the photo-detectors 11 and 12 in a predetermined spot size. A halfwavelength plate 13 is positioned between the beam splitter 9 and thespot lens 10 for rotating the plane of the polarization of theinformation light in a direction intermediate between the S-axis andP-axis of a polarizing beam splitter 14 which acts as an analyzer.Reference numeral 15 represents a spot lens, and reference numeral 16represents a cylindrical lens so disposed as to have its focal lineinclined 45° relative to the direction of connection of the guidegrooves of the magneto-optic disc 1.

Positioned on one side of the cylindrical lens 16 opposite to a spotlens 15 is the photo-detector 17 of the composite element type whichincludes six photo-detector elements A to F that are arranged in apattern as shown in FIG. 2. By the cumulative effects of the spot lens15 and the cylindrical lens 16, both the change in relative distancebetween the magnetizable layer of the magneto-optic disc 1 and theobjective lens 8, and the deviation of position of the light spotbetween the magnetizeable layer and the information track (guidegrooves) can be detected. A method of detecting these servo signals issimilar to that used in any of the astigmatism method, the three-beammethod and the push-pull method, all generally used in association withthe optical disc.

FIG. 2 illustrates the relationship in position between thephoto-detector 17 of the composite element type and the reflectedinformation light from the magneto-optic disc 1, and also a detectingmeans. Based on respective outputs SA, SB, SC and SD of four lightreceiving elements A, B, C and D positioned centrally of the detectingmeans 17, a focusing error signal F_(E) and a main beam tracking errorsignal T_(E0) can be obtained from the following equations.

    F.sub.E =(SA+SC)-(SB+SD)

    T.sub.E0 =(SC+SD)-(SA+SB)

Also, based on respective outputs of light receiving elements E and Fpositioned on respective sides of the four light receiving elements A,B, C and D, a tracking error signal T_(E1) resulting from two auxiliarybeams can be obtained from the following equation.

    T.sub.E1 =SE-SF

Of these error signals, the focusing error signal F_(E) is used tocontrol the drive of the objective lens 8 in a direction parallel to theoptical axis which is effected by the servo control circuit and a drivedevice.

On the other hand, the main beam tracking error signal T_(E0) and thetracking error signal T_(E1) resulting from the two auxiliary beams are,after having been processed by some processes as will be describedlater, used to control the drive of the object lens 8 in a directiontransverse to the track which is effected by the servo control circuitand the drive device.

Hereinafter, the processes necessary to effect the tracking by the useof the above-mentioned two tracking error signals will be described.

FIG. 3 is a circuit block diagram used to explain the first process.Reference numeral 18 represents a low pass filter unit for obtaining alow frequency component of the main beam tracking error signal T_(E0).The low frequency component filtered through the low pass filter unit 18is fed to a sample-and-hold circuit 19 for sampling and holding theoutput from the filter unit 18 in synchronism with a timing signal aswill be described later. Reference numeral 20 represents a differentialcircuit for outputting a difference signal indicative of the differencebetween the output from the sample-and-hold circuit 19 and the main beamtracking error signal T_(E0), said difference signal being in turn fedto a switching circuit 21 which is operable to switch between thetracking error signal T_(E1) and the tracking error signal output fromthe differential circuit 20 in synchronism with the timing signal. Theswitching operation of the switching circuit 21 is performed by theutilization of timing signal, the waveform of which is shown by (a) inFIG. 4, in such a way that, at regions other than the address regionsand the boundary regions, tracking can be carried out according to theTwin Spot method (i.e., the signal T_(E1)) wherein a stable trackingerror signal can be obtained, but at the address regions and theboundary regions, the amount of offset of the main beam tracking errorsignal T_(E0) can be monitored during tracking according to the TwinSpot method, and tracking can be carried out according to the push-pullmethod (i.e., the output of the differential circuit 20) by the mainbeam with the offset taken as a target value. Also, by the utilizationof the timing signal shown by (b) in FIG. 4, the push-pull method (i.e.,the output of the differential circuit 20) can be utilized for trackingonly at the boundary regions where the error signal (i.e., the signalT_(E1)) tends to be disturbed according to the Twin Spot method.

As a second process which can bring about a similar effect, a method canbe used wherein either the error signal associated with the auxiliarybeams or the main beam is monitored while the tracking is normallycarried out according to the Twin Spot method, but the tracking at theaddress regions and the boundary regions is carried out by holding thetracking error signal T_(E1) according to the Twin Spot method or a lowpass filter output signal of the push-pull signal T_(E0) associated withthe main beam. This process can work very well where the time requiredto scan the address regions is short and the amount of movement oftracks during this time is also small.

Although not discussed, care must be taken that the sensitivity ofdetection of various tracking error signals must be optically orelectrically consistent and that any change in output incident to achange in power of the laser during recording and erasure must becontrolled by any suitable method, for example, by the use of anautomatic gain control circuit.

The tracking system according to the foregoing embodiment isadvantageous in that, since the advantage of Twin Spot method isutilized, the stable tracking can take place even on the optical dischaving the address information partially preformatted.

The tracking system according to a different embodiment of the presentinvention will now be described with particular reference to FIGS. 5 to8.

In FIG. 5, which illustrates the construction of an optical head for usewith an optical disc to which the present invention applies, referencenumeral 100 represents the optical disc having permanently formedtherein non-continuous guide grooves representative of the previouslydiscussed guide grooves and address information. Reference numeral 102represents a laser device for producing a predetermined laser beam.Reference numeral 103 represents a diffraction grating for producing amain beam used to carry out the recording, reproduction or erasing, aswell as auxiliary beams used to obtain auxiliary tracking error signals,said grating having a direction of diffraction located on respectivesides of a recording track. Reference numeral 104 represents a beamsplitter operable to pass a portion of the incoming laser beamtherethrough and also to guide the laser beam, which is reflected fromthe optical disc 100, towards a photo-detector 108. That portion of thelaser beam which has passed through the beam splitter 104 is formed byan objective lens 105 into a tiny spot of light which is projected ontothe optical disc 100. This objective lens 105 can be controlled by aservo control circuit and a lens drive circuit, both not shown, so as tobe moved in a direction parallel to the optical axis and also in adirection perpendicular to the tracks. In this way, the size and theposition of the spot of the laser light projected onto the optical disc100 can be adjusted.

The laser beam which has been reflected from the optical disc 100 andguided by the beam splitter 104 travels towards the photo-detector 108of the composite element type through a spot lens 106 and then through acylindrical lens 107 so positioned as to have its focal line inclined45° relative to the direction of connection of the guide grooves of theoptical disc 100. The relationship between the position of the spot ofthe laser beam projected onto the optical disc 1, the arrangement ofdetecting elements A to F of the photo-detector 108, and the position ofthe spot of the reflected light on each detecting element is shown inFIG. 6.

Referring to FIG. 6, reference numeral 100' represents a recording trackformed on the optical disc 1, reference character a represents the spotof the main beam of the incident laser projected on the recording track100', and reference characters b and c represent respective spots of theauxiliary beams. Reference character a' represents a spot formed by areflected light of the main beam, whereas reference characters b' and c'represent respective spots formed by reflected light of the auxiliarybeams. In the construction shown therein, a focusing error signal F_(E)necessary to move the objective lens 105 in the direction parallel tothe optical axis, and tracking error signals T_(E0) and T_(E1) necessaryto move the objective lens 105 in the direction perpendicular to thetracks can be obtained according to the astigmatism system, thepush-pull system and the three-beam system, respectively, and can bedetermined by performing calculation according to the followingequations.

    F.sub.E =(SA+SC)-(SB+SD)

    T.sub.E0 =(SC+SD)-(SA+SB)

    T.sub.E1 =SE-SF

wherein SA, SB, SC, SD, SE and SF represent detection signals outputtedfrom the detecting elements, respectively.

FIG. 7 illustrates a circuit block diagram showing a processing circuitfor producing a feedback control signal on the basis of the trackingerror signals T_(E0) and T_(E1) referred to above. Reference numeral 109represents a low pass filter for extracting only a low frequencycomponent of the tracking error signal T_(E1) according to the Twin Spotmethod, and reference numeral 110 represents a differential circuit forperforming addition or subtraction between the output of the low passfilter 109 and the tracking error signal T_(E0) according to thepush-pull system. Whether the differential circuit 110 performs additionor whether it performs subtraction depends on the relationship in phasebetween the error signals T_(E1) and T_(E0). By way of example, when theerror signal T_(E0) according to the push-pull system matches in phasewith the error signal T_(E1) according to the Twin Spot system as shownin FIG. 8 at the time an offset occurs in the spot of the incident laserbeam, an offset occurring in the error signal T_(E0) can be compensatedfor by adding the low frequency component, indicated by Δ in FIG. 8(b),of the error signal T_(E1) produced incident to the tracking to theerror signal T_(E0), and therefore, accurate tracking can be achieved ascan be understood from FIG. 8(b).

An output from the differential circuit 110 is fed to a switchingcircuit provided for enabling a stabilized withdrawal of the tracking,which circuit 111 can enable the tracking error signal T_(E1) accordingto the Twin Spot system to be utilized in the event that no steadyoffset occurs in the error signal at the time of withdrawal from thetrack, but the tracking error signal which has been processed in themanner as hereinabove described can be utilized subsequent to thewithdrawal.

The processing circuit described with reference to and shown in FIG. 7can be modified as shown in FIG. 9 wherein like parts shown in FIG. 9are designated by like reference numerals used in FIG. 7. Referencenumerals 112 and 113 represent differential calculators, respectively,each of said calculators being similar to the differential calculator110 shown in FIG. 7. This circuit shown in FIG. 9 is so designed that,where the error signal T_(E0) according to the push-pull system and theerror signal T_(E1) according to the Twin Spot system are used to obtaina signal S' from a combination of the calculator 112 and the low passcircuit 109 as shown in FIG. 10(b), a difference signal S representativeof the difference between the signal S' and the error signal T_(E0) canbe obtained from the calculator 113 as shown in FIG. 10(c). By theutilization of this difference signal S, accurate tracking can beaccomplished.

Thus, by the use of the foregoing technique according to the presentinvention, a tracking error signal for tracking control can beautomatically corrected even though an offset may occur in the push-pullsignal, and therefore, stable tracking can be accomplished. In addition,since a reduced component of the tracking error signal according to theTwin Spot system is utilized, any external disturbance which would occurduring the tracking at the boundary between the recorded or non-recordedarea or the guide grooves and address region can be minimized to anegligible amount.

From the foregoing, it is clear that the present invention is effectiveto provide a tracking system which utilizes the advantages of both theTwin Spot system and the push-pull system. Moreover, even with anoptical disc having guide grooves and address regions as well as anoptical disc of a type which would result in change of reflectivity, thetracking system of the present invention can ensure a sufficientlystabilized tracking performance.

Although the present invention has been fully described by way ofexample with reference to the accompanying drawings, it is to be notedhere that various changes and modifications are apparent to thoseskilled in the art. Such changes and modifications are to be understoodas included within the scope of the present invention, unless theydepart therefrom.

What is claimed is:
 1. A tracking system for an optical memory dischaving a plurality of recording tracks thereon including track addressregions and track information regions comprising:an optical headassembly including light beam producing means for producing a main lightbeam and two auxiliary light beams, said main and auxiliary beams beingscanned over said plurality of tracks; detector means for detectinglight beams reflected from said tracks as a result of said scanning andproducing a first tracking error signal in response to the lightreflected from said main beam and a second tracking error signal inresponse to the light reflected from said two auxiliary beams; andtracking control means for performing tracking control of said main beamon one of said tracks in response to said first tracking error signal ata boundary between a track address region and a track informationregion, and said second tracking error signal at track regions otherthan said boundary.
 2. The tracking system defined in claim 1, whereinsaid tracking control means comprises:sample-and-hold means for holdingsaid first tracking error signal during scanning of said boundary;differential circuit means, having inputs coupled to receive said firsttracking error signal and the signal held by said sample-and-hold means,for providing a difference signal representing the difference betweensaid first tracking error signal and the signal held by saidsample-and-hold means; and switch means, having inputs coupled toreceive said difference signal and said second tracking error signal,for providing either said second tracking error signal or saiddifference signal at an output thereof, dependent on whether said systemis scanning said boundary or not.